In the prior art self-adhering packaging films which are heat shrinkable are well known; and, likewise, heat shrinkable abuse resistant films which are relatively impervious to the passage of gas or moisture are also well known. Accordingly, a prime object of the subject invention is to combine the best qualities of both of these types of films into a single film or film laminate.
In making a self-welding film, particularly a self-welding film which is to be made into receptacles such as bags, three key factors must be balanced, namely: processability, sealability and seal strength, and self-welding ability. Processability concerns the ease with which a bag or receptacle may be made from film free of wrinkles and creases and, after being made, the ease with which a bag or receptacle may be opened and filled. Sealability concerns the relative speed of making bags and the strength of the heat seal which closes one end of the bag when the bag is made from flattened tubing. This primary closure seal is usually made with heated jaws under pressure. Self-welding ability concerns the ability of the material to adhere to itself at elevated temperatures. Therefore, it is another object of the present invention to provide a self-welding film which has an optimum combination of processability, sealability, and self-welding characteristics.
A typical prior art self-welding film is disclosed in U.S. Pat. No. 3,625,348 which issued on Dec. 7, 1971 to Oliver R. Titchenal et al. Set forth in the Titchenal et al patent is a bag made from a heat shrinkable multi-layer plastic film which has an inner layer of ethylene-vinyl acetate (also designated herein as "EVA" with accompanying percentage representing vinyl acetate content by weight) copolymer laminated to a layer of vinylidene chloride-vinyl chloride copolymer and then biaxially oriented. When an article is placed in the bag and the bag submerged in a hot water bath, the bag will shrink into conforming contact with the article and all the opposing contacting surfaces of the EVA copolymer layers will seal to each other. An advantage of such a bag structure is that upon puncture of the bag the multi-layer film remains in conforming contact with the article. However, the abuse resistance and strength of these prior art packages is not completely satisfactory for normal commercial usage where packages of large cuts of meat or large chunks of cheese will be transported and receive rough handling. Thus, it is an object of the present invention to provide an abuse resistant, self-welding packaging film which significantly improves package integrity.
In U.S. Pat. No. 3,741,253 which issued on June 26, 1973 to Harri J. Brax et al a packaging film having low oxygen permeability is disclosed. This packaging film is a three layer laminate having an inner layer of cross-linked EVA copolymer, a middle layer of vinylidene chloride copolymer, and an outer layer of an EVA copolymer. This packaging film has excellent abuse resistance but because the inner layer is irradiated or cross-linked to improve its strength and abuse resistance, it is limited in self-welding ability. Accordingly, it is another object of the present invention to provide an irradiated or cross-linked polymeric material which has superior self-adhering or self-welding characteristics.
As mentioned in the foregoing paragraphs, self-welding packaging films are most often made into pouches, bags, containers, or receptacles. For packaging fresh meats it is desirable that the container not only be self-welding but that it have low gas and moisture permeability and be heat shrinkable and abuse resistant because current packaging techniques require that fresh beef, for example, remain out of contact with air if it is to be stored chilled and unfrozen for periods in excess of 10 days. For such storage, beef products are preferably placed in a self-welding container, the container evacuated, and sealed with a heat seal or metal clip. After sealing, the bag is shrunk around the enclosed product by placing it in an elevated temperature medium such as a heat tunnel or hot water bath. In the self-welding bag or container the excess areas of the bag "weld" together after the bag has been shrunk. Self-welding provides a second seal should the clip or heat seal fail or the bag be punctured and the self-welding also will retard movement and seepage of blood and juices or "purge" within the package because the container material has welded to itself and there are no wrinkles or crevices in the material into which the purge can seep. Thus, an advantage to the present invention is that there is less loss due to defective clips or seals and the packaged product has an improved appearance by preventing the seepage of the juices into the unshrunk and unsealed areas of the bag.
The term "self-welding" is not defined in precise, measurable parameters but is a subjective term, and, a "self-welding" material is best defined as one which will adhere to itself in a contact seal and will stay welded through normal commercial distribution without external force so that the self-welded areas remain impenetrable to purge. The expression "without external force" means that the self-adherence or contact weld is maintained by the strength of the weld alone and is not due to any extraneous force pushing the film sheets or package walls together as would be the case when the interior of a bag is evacuated so that the bag walls collapse against themselves and are held in contact under the influence of atmospheric pressure.
As will be readily apparent to those skilled in the art the foregoing objects are accomplished by the present invention which is summarized below.